collectd
A daemon that collects system performance statistics periodically, and provides tons of plugins to store and analyze these values
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Nagios Core
Nagios Core network monitoring software is a powerful, enterprise-class host, server, application, monitoring and alerting tool
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Zabbix
An open source, enterprise-class monitoring framework
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Zipkin
A distributed request tracking system, useful for micro-service and distributed architectures written in Scala
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